Specifications

Ref# 420826 Intel
®
Atom™ processor CE4100 137
Platform Design Guide
Intel Confidential
10.3 VCXO Mode Design Guidelines
The Voltage-controlled Crystal Oscillator’s (VCXO) output frequency changes in proportion to
the application of any input control voltage. The VCXO is used to provide synchronization of
the input Transport Stream (TS) to the AV output. The rendered frame rate of the AV must
match the frame rate of the input TS to prevent frame drops or duplications and the audio
and video defects associated with them. This VCXO must have sufficient accuracy and pull
ability to be able to lock to the reference, and Absolute Pull Range is the measure of that
ability.
This kind of VCXO has internal tunable shunt capacitors. On-board capacitors should be
unusually small or completely absent to let the internal capacitors do the tuning. The typical
range of tuning from VCXO_CNTL software control should be around +/- 120ppm.
Intel strongly recommends using a pullable crystal for the VCXO clock source. The board
must be voided out underneath the crystal in order to get the VCXO frequency above 27MHz.
The crystal traces should include pads for small fixed capacitors, one between X1 pin and
ground, and another between X2 pin and ground. Stuffing of these capacitors on the PCB
might be optional. The typical value might be 1pf to 4pf. The need for these capacitors is
determined by the system prototype evaluation and is influenced by the particular crystal
used and PCB layout. Intel recommends removing the shunt capacitors on the crystal at the
first testing.
Note
The crystal used should be pullable. If the crystal type is not correct, the tuning range
might be substantially lower than expected (+/-100PPM). As MEMS oscillators may be
very difficult to pull.
The VCXO_CNTL is the signal used by software to tune 27MHz AVPLL input. This pin has an
internal digital sigma-delta DAC implementation and need to have an external filter before
connecting to an external VCXO.
• 3.3 KΩ Resistor, 1%
• C1/C2/C3 capacitors, ESR<0.3, ESL<25nH
• C1= 1uF; C2 = 0.1uF; C3= 0.01uF